[identity] fix CORS

Summary: i was going to manually implement a CORS layer on our service, but tonic-web just added a much easier way to enable CORS in v0.9.1

Test Plan: followed the steps here and got back an OK response on web for the GenerateNonce RPC

Reviewers: ashoat, jon

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7307